CHAPTER XVI 
THE SOIL 
DeriniTion AND Funcrion—The soil is the layer of decom- 
posed rock material charged with organic matter which 
covers most of the surface of the earth. It forms the basis 
of vegetation as it can be penetrated by roots and holds 
stores of plant foods. The study of soils reveals the causes 
of their fertility or sterility, and has enabled large tracts of 
useless desert to be rendered of high fertility. The soil is 
usually brownish or black, and from 6 inches to 2 feet in 
thickness ; it passes into the underlying rock through the 
partially decomposed layer, the subsoil—a term however 
used in civil engineering and public health for the general 
foundation of a district. 
Soils are of two chief classes, * sedentary soils ” due to 
the weathering of rock in situ, and * transported soils 
composed of materials that have undergone disintegration 
elsewhere. The decay of rock into soil is due to changes of 
temperature which break rock to pieces, solution by soil 
waters, oxidation, the burrowing of animals, the disruptive 
action of roots, and the solvent action of organic products. 
The main function of the soil is to convert the inorganic 
constituents of the air into plant tissues which can be used 
as food by animals. A soil is also a reservoir of plant foods, 
which in some circumstances may be its main value, for the 
deep soils of Manitoba, which contain 20 per cent. of organic 
matter, can produce wheat for a long period without exhaus- 
tion; but if a soil be worked simply as a store of plant food, 
it must be ultimately ruined. 
The value of a soil may depend on accidental circumstances, 
such as proximity to a market, local supplies of cheap manure, 
and freaks of fashion, which render profitable the growth of 
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